The present invention relates generally to imaging systems and, more particularly, to digital detectors. Even more particularly, the present invention relates to a novel technique for protecting and resetting a detector by self cycling read/scrub sequences.
Digital imaging systems are becoming increasingly widespread for producing digital data, which can be reconstructed into useful images. In current digital imaging systems, such as digital X-ray systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons, which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient""s tissues similar to those available through conventional photographic film techniques.
In available digital detectors, the surface of the digital detector is divided into a matrix of picture elements or pixels, with rows and columns of pixels being organized adjacent to one another to form the overall image area. When the detector is exposed to radiation, photons impact a scintillator coextensive with the image area. A series of detector elements are formed at row and column crossing points, each crossing point corresponding to a pixel making up the image matrix. In one type of detector, each element consists of a photodiode and a thin film transistor. The cathode of the diode is connected to the source of the transistor, and the anodes of all diodes are connected to a negative bias voltage. The gates of the transistors in a row are connected together and the row electrode is connected to scanning electronics. The drains of the transistors in each column are connected together and each column electrode is connected to additional readout electronics. Sequential scanning of the rows and simultaneous read out of the signal from the columns permits the system to acquire the entire array or matrix of signals for subsequent signal processing and display.
In use, the signals generated at the pixel locations of the detector are sampled and digitized. The digital values are transmitted to processing circuitry where they are filtered, scaled, and further processed to produce the image data set. The data set may then be used to store the resulting image, to display the image, such as on a computer monitor, to transfer the image to conventional photographic film, and so forth. In the medical imaging field, such images are used by attending physicians and radiologists in evaluating the physical conditions of a patient and diagnosing disease and trauma.
Typically, the read out of a solid state X-ray detector is initiated by the X-ray system. To minimize the effects of leakage, the detector is reset by special commands from a controller by reading the pixel circuits without collecting the data (sometimes referred to as xe2x80x9cscrubbingxe2x80x9d) at some optimized rate. Normally, the system should read the detector yet not collect the data on a routine basis. However if a failure occurs at the system level, the system may fail to read or scrub the detector for very long periods. The effect of such a problem is that when thin film transistors (TFTs), which are switches that isolate the photodiodes, are not operated for long periods of time, their thresholds begin to shift. If the thresholds shift too severely, the TFTs no longer provide sufficient isolation, and the signal from each pixel may begin to leak. This results in the inability of the readout electronics to discern the desired exposure data on a pixel by pixel basis, and corrupts the image data.
A similar problem arises when, during the course of manufacturing, a detector is powered up without being attached to an X-ray system. If the detector is maintained under bias for extended periods without being attached to the system and therefore without being read or scrubbed, the threshold may shift and the detector may be damaged beyond reclamation.
There is need, therefore, for an improved technique for resetting digital detectors via self initiated reading or scrubbing operations preformed under a defined set of conditions.
The invention provides a technique for resetting charge on the rows and columns of pixels in a digital detector. The technique may be employed in both newly designed imaging systems, or may be retrofitted to systems to upgrade the image quality and to provide enhanced protection for the detector circuitry. The technique offers an automatic reset circuit which may be interconnected to a communication circuit to route both incoming and outgoing data between the detector and the control circuit. The automatic reset circuit may comprise various electronics to control the automatic reading of the detector. The reset circuit, for example, may include a receive state machine, a timer and a read out state machine and so forth. The system may be integrated with various resetting functions to further facilitate the efficiency and protect the image quality of the detector. Thus, monitoring functions, and commanding reset functions, may be integrated within one circuit.